1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method for generating image data in the prepress process, and relates more specifically to the process of extracting adjoining vectors identifying locations where trapping images should be generated to prevent exposing the underlying part at color boundaries in color printing.
2. Description of Related Art
A computer called a front-end processor is used in prepress processes to produce, edit, and layout the text, logos, images, pictures, illustrations, and other numerous data parts composing the printed product, and thus output edited layout data (“page description data” below) representing the printed output in a page description language. The edited layout data, i.e., the page description data, is then rasterized by a raster image processor (RIP) to produce bitmapped image data representing the image to be printed. Printing plates are then produced by a plate making machine using this bitmapped image data, or a digital printer prints the bitmapped image data directly to hard copy output.
Before the RIP rasterizes the page description data outputted by the front-end processor, registration marks are added and a trapping process is applied to the page description data. Processes performed before processing by the RIP are referred to herein as RIP preprocesses.
This trapping process spreads the lighter (paler) of two overlapping colors in order to prevent exposing the base (the part knocked out) at the color border due to misregistration during color printing. A prerequisite of this trapping process is detecting where trapping is required using a method such as taught in U.S. Pat. No. 6,577,777, which is incorporated herein by reference and corresponds to Japanese Non-examined Patent Publication H09-106460.
To detect where trapping is required, adjoining vectors such as described below are extracted in RIP preprocessing. Specifically, when two objects, which are parts (shapes) of the overall image in the printout represented by the page description data, are overlapping, the vectors indicating the border between the two objects are extracted as adjoining vectors. More specifically, the objects constituting a page are in an upper-lower relationship based on the order in which they are formed on the page. When two objects overlap, and the part of the contour of the upper object that is equivalent to the path between the intersections of the contours of the two objects is contained in the area of the lower object, that part is extracted as an adjoining vector. An overprinting outline (stroke) with a specified trapping width is then generated along this adjoining vector as the trapping image.
There are also cases in which a slight gap is present because there is little overlap between two very close objects. Trapping is also required in such cases. Depending on how the adjoining vectors are extracted, however, adjoining vectors cannot be extracted for this gap, and trapping cannot be applied where needed. When two curves in the contours of two objects P6 and P7 are tangent at two points as shown in FIG. 7, for example, adjoining vectors are not extracted between these two points, and the gap region Rg shown in FIG. 7 cannot be trapped.